Here are a selection of online resources that I use when developing. Some I use far too often, and some I have to say I use far too little!
This list is by no means comprehensive, and I will be updating it over time.
I am always on the look out for new sites like these, so please feel free to comment this post with your suggestions.
codingforums.com
I am a bit of a sucker for forums and have found this one to have a good bunch of people behind it, they are willing to help and very welcoming.
php.net
Self explanatory really, the docs on this site tell you how each and every PHP method should be used with some excellent community based examples.
stackoverflow.com
Think of a free version of Experts Exchange aimed directly at programmers and about 10 times better, that is what Stack Overflow is. Sign up for a free account and ask questions (from the very very simple, to the very very complex, all levels are catered for).
tizag.com
A very basic tutorial website covering a range of languages, for when you have a bit of a brain freeze and know the solution is way too simple to ask for help!
validator.w3.org
Valid HTML is happy HTML (you can also validate your CSS there too).
w3schools.com
Much like Tizag only much more established.